Many coaxial cables are required to be used in the high RF fields used in MRI. These include primarily the cables to the receive coil array but also other cables that must enter the high RF field such as those used in pacemakers, ECG testing, electrophysiology and EEG monitoring, and Deep Brain Stimulation systems (DBS).
Common mode signals or shield currents on coil cables are often caused by the coil itself or by an external source such as a surrounding transmit body coil during transmit phase. Electromagnetically induced currents by an external source, such as those produced by the body transmit coil, are responsible for the majority of the shield current and therefore heat, on the surface of the cable. These currents, and the resulting heat produced, can cause serious patient heating or burns. Common mode currents also degrade the image quality by affecting coil tuning, coil-to-coil coupling in phased array coils.
In addition the generation of currents in the shield of cables within the coil, especially cables close to or crossing the individual coil loops in a phased array coil, in the magnetic field of the MR scanner can interfere with the creation of the homogenous RF field generated by the transmit body coil. This inhomogeneity of the RF field can generate artifacts within the image obtained.
The advantageous use of coaxial cables having an inner axially oriented elongated conductor separated from an annular electrically conductive shield by a dielectric material has long been known. Such coaxial cables have been used in magnetic resonance imaging, as well as numerous other uses.
Among the important safety concerns related to magnetic resonance imaging technology are the possible burns and excessive heat due to the induced RF currents on the electrical cables. To reduce the risk of such localized heating or burns, the users of the MR scanners are instructed to minimize patient contact with cables. Such contact, however, is unavoidable in many cases such as when using ECG cables, surface coils, or intra-cavity coils.
To minimize localized heating or burns and induced currents on the cables, some commercial MR coils, such as the magnetic resonance coils of GE Medical Systems, for example, use patient safety modules. This design decreases the unbalanced currents on the coaxial cable. In addition to patient safety, this design effects reduction in radiation losses and common mode noise in the coil.
Similar and more serious problems exist for the coils that are inserted inside the body such as endorectal, esophageal, and intravascular RF probes. As these devices are closer to the body, the risk of localized heating or burning a patient is increased. Also, the wavelength of the RF signal in the body is approximately nine times shorter as compared with the wavelength in the air. As a result, current induction on short cables is possible. There remains, therefore, a need for an improved coaxial cable which will perform effectively for its intended purpose while resisting the generation of high currents in the shield which can cause undesired excessive heating or burning of a patient and which can cause interference with the homogeneity of the RF field thus generating artefacts.
Typically the effect of the generation of currents in the shield of the coaxial cable is reduced by using cable traps which are placed in the cable at spaced positions along the length of the cable. These act to reduce the generation of the current.
This is particularly exacerbated where the cable must be very long to accommodate various movements, such as in the system described in U.S. Pat. No. 5,735,278 (Hoult et al) issued Apr. 7, 1998 in which is disclosed a medical procedure where a magnet is movable relative to a patient and relative to other components of the system. The moving magnet system allows intra-operative MRI imaging to occur more easily in neurosurgery patients, and has additional applications for liver, breast, spine and cardiac surgery patients. In this case the high number of cable traps required in the intra-operative MRI coil signal transmission cable in conjunction with the great length of the cable makes the cable particularly unwieldy.
One type of cable traps typically involve an inductor formed from the cable shield braid by wrapping the cable around a helical support so that the shield forms a helical inductor. At one end the copper conductor is electrically connected to the cable shield braid and at the other end one or more capacitors are connected in parallel to the inductor between the copper conductor and the shield to form a tank circuit which acts to attenuate the unwanted shield current on the cable.
In the cable trap arrangement, the shield braid is continuous along the cable and has formed at points along its length the tank circuit defined by the inductor portion of the shield, the copper conductor, and the capacitors.
The cable traps improve the coil performance by eliminating or reducing the shield current along the cable shield. The cable trap is designed to reduce the shield current, but the helical inductor formed from the cable shield of the cable trap also effectively acts as an antenna, to receive RF power from the transmit body coil and contributes unexpected current in the cable.
Experiments have shown that the copper conductor contributed additional heat. This type of cable trap increases the overall coil and cable weight and is not convenient for handling in a surgical setting.
The generation of the shield current is proportional to the geometry of the cable. A larger surface cable generates more shield current than a smaller surface area cable. For example, a longer cable with a larger diameter produces more current than a shorter cable with a smaller diameter.
The generation of the shield current is also proportional to the system RF power. For example, the power from a 3.0 Tesla system will be four times the power from the 1.5 Tesla system, and much higher power for a 7.0 Tesla or higher system. The required number of cable traps for a 3.0 Tesla system will be approximately doubled compared to the 1.5 T system, with closer spacing between cable traps. A 7.0 Tesla or higher system would require even more cable traps with closer spacing.
Also the additional length of the raw cable required, when wrapped helically, to form a cable trap negatively affects the RF chain.
A number of cable designs have previously been proposed as follows:
U.S. Pat. No. 6,284,971 (Atalar) issued to Johns Hopkins University on Sep. 4, 2001 discloses a co-axial cable for probes used in MRI, which has an outer dielectric layer with high dielectric constant, between inner shield portion and a segmented outer shield portion of outer conductor so as to inhibit induced radio frequency current. Thus the arrangement disclosed connects the one end of a segmented shield to the cable shield braid and use the free end of the segmented shield as a ¼ wave cable trap.
U.S. Pat. No. 7,123,013 (Gray) issued to Biophan technologies on Oct. 17, 2006 discloses an arrangement in which a voltage compensation unit reduces the effects of induced voltages upon a device having a single wire line having balanced characteristic impedance. The voltage compensation unit includes a tuneable compensation circuit connected to the wire line which applies supplemental impedance to the wire line and causes the characteristic impedance of the wire line to become unbalanced, thereby reducing the effects of induced voltages.
U.S. Pat. No. 7,205,768 (Schulz) issued to Phillips on Apr. 17, 2007 discloses a lead for use in an MRI device which has an auxiliary electrical device connecting to the lead with sections with inductive coupling element of limited length not equal to integral multiple of the half wavelength.
U.S. Pat. No. 7,294,785 (Uutela) issued to GE Healthcare on Nov. 13, 2007 discloses a lead for use in an MRI device where, in order to eliminate the risk of thermal injuries without compromising the signal-to-noise ratio more than what is required for patient safety, the lead comprises two successive cable elements having different resistance characteristics. The second cable element, which is connected by the first cable element to the patient, has a total resistance increased from a normal high-conductivity resistance value of a patient cable to suppress antenna resonances in the second cable element. The first cable element, which is connected to the electrodes on the skin of the patient, has a total resistance substantially greater than that of the second cable element to prevent electromagnetically induced currents from flowing to the patient and to prevent excessive heating of the cable by electromagnetic induction.